Room: Exhibit Hall | Forum 2
Purpose: To evaluate dose calculation accuracy of optimized pencil beam scanning (PBS) plans on MR-derived synthetic-CTs for prostate patients.
Methods: Ten patient datasets with both a CT and an MRI were planned with opposed lateral proton beams optimized to single field uniform dose under an IRB approved study. The plans were performed on datasets generated by a commercial synthetic CT-based software called MRCAT (MR for Calculating ATtenuation) routinely used in our clinic for photon-based planning. The CTV was expanded to 2 mm posteriorly, and 3 mm elsewhere to create the PTV. A PTVeval volume was created by expanding PTV laterally by 5mm to take into account proton range uncertainty. A standard prescription of 79.2 Gy (RBE) and 68.4 Gy (RBE) was used for intact prostate and prostate bed cases. Proton plans were then recalculated on a deformed-CT, deformed to match outer contour between MRCAT-CT and original CT. The same beam arrangement, total MUs, MUs/spot, spot positions were used to recalculate dose on deformed-CT without re-normalization. DVH analysis compared D98% for PTVeval, CTV, and V70Gy, V50Gy for OARs. Gamma analysis using (2%/2mm) was performed between the MRCAT-CT and deformed-CT.
Results: The percentage dose difference between MRCAT-CT and deformed-CT, were (1.14 Â± 2.37 %) for PTVeval D98% and (0.04Â±0.06%) for CTV D98%. Rectum V70Gy, V50Gy, and Bladder V70Gy were (2.76Â±4.01 %), (11.6Â±11.2 %), and (3.41Â±2.86 %) respectively for the MRCAT CT, and (3.23Â±3.63 %), (11.3Â±8.18 %), and (3.29Â±2.76 %) respectively for the deformed-CT. The average gamma pass rate using (2%/2mm)10% between MRCAT-CT and deformed-CT were 99.3 Â±1.1 %.
Conclusion: Dosimetric analysis shows that MR-only proton planning is feasible using MR-derived MRCAT synthetic-CT based on current clinical margins that account for a range uncertainty. With multi-spectral CT, the accuracy of synthetic-CT for PBS will need to be further evaluated.
Funding Support, Disclosures, and Conflict of Interest: Memorial Sloan Kettering Cancer Center has a master research agreement with Philips Healthcare.